Catenary - would you buy krypton piano wire?

[thinwater]

Digressing slightly back to the original topic, if the catenary disappears under any significant strain, is there actually any need to put out more scope with rode over chain?

Not to be fussy, but the rode is the line attaching the boat to the anchor. It can be either chain or rope. Referring to a rope rode as... rode, is confusing.

(Websters Dictionary) Definition of rode
: a line (as of rope or chain) used to attach an anchor to a boat

As for whether you need more scope with rope, this is an anchoring thread, so abandon all hope of a straight answer:ambivalence:.

The approximate answer is that you need ~ 10:1 scope, just as you would for chain in relatively shallow water, because in a strong wind they amount to the same thing. It is seldom good practice to anchor at short scope with rope, even in moderate weather, because there will be up-lift. In deeper water (>20 feet) chain maintains some curve even in rather strong winds and somewhat shorter scope is practical, as it is in lighter winds. But with rope it is generally at least 7:1 scope all the time, and some times >10:1 scope in shallow water. All of this depends onthe anchor and the bottom as well; a modern anchor in good sand or mud can be quite capable at shorter scope, but at some point, it is a gamble.

So yes, more scope in light winds (when chain can go short), and maybe just little more in strong winds.

 

[[163233]]

Not to be fussy, but the rode is the line attaching the boat to the anchor. It can be either chain or rope. Referring to a rope rode as... rode, is confusing.

Yes of course, but you knew what I meant..

The approximate answer is that you need ~ 10:1 scope...

The RYA suggest minimum 4x for chain and 6x for rope, which is why I was wondering. From what's been said the additional for rope shouldn't really matter.
Sailtrain repeat that here:- http://www.sailtrain.co.uk/anchoring/anchoring.htm

10x sounds a lot I'm not sure my lockers would cope.. Might be viable somewhere with not much tide.

 

[RupertW]

Over about 8:1 scope rocna reckon not -
http://kb.rocna.com/kb/Scope_vs_catenary

Interesting questions though, could loads of scope make matters worse by allowing the boat to speed up more when it tacks about in the big gusts? Would a riding sail & some buckets off the bow actually help more by keeping the velocity down as the energy the boat has is proportion to speed squared?

Even answerable?

Anyone have any guesses how much worse riding about the anchor is on a skinny chain compared to more normal size?

I think the long scope and veering is a very interesting question - there's quite a bit of momentum on our boat built up on a swing if we are at 5 or 8 to one compared to being in a crowded anchorage at 3:1 in a blow. In both cases we've never dragged so far but I have felt more nervous at the shock loads even with snubbers. This year I'm planning to use forks a bit more in those situations but if I'm the only person doing that then I could hit nearby boats on one anchor alone.

I don't think the light chains would make a difference for movement except in less than maybe 5 knots of wind as the slightest breeze will move our 10mm chain across the top of a sandy bottom. A gloopy mud one might be different.

 

[thinwater]

a. I warned JBJag not to expect a straight answer!

b. If you have a lot of tide, you have both the big scope shallow water case and the shorter scope deeper water case; there is no conflict here.

c. I would avoid Sailtrain. That advice was very optimistic. With good mud and settled weather you can get away with short scope, but in other bottoms and more wind, I watched a lot of people drag when a thunderstorm showed up. It also depends on the anchor type.

d. Horsing (surging). In fact, I've been doing some testing on the subjects of yawing and horsing for some up-coming articles. You can Google it, of course. In light winds, chain helps, but once it starts kicking, it's all about windage and underbody. Those are the real forces. Chain vs. rope is pretty trivial, since most of the chain is off the bottom and chain the same size as rope (drag through water). Second, you actually don't surge much more on rope than chain (I've switched back and forth). The chain has some caternary stretch plus snubber. The rope includes 30 feet of chain, so it isn't all rope. The difference in stretch is 1-2 feet, so on the same boat, all things being equal (that is what I was testing), not much difference. We hear of and see big differences, but there were probably other factors. For example, boats with rope tend to be fin keel designs with a big main and the keel well behind the mast. I watched one sailing like mad the other day; he had a dinghy on the foredeck catching wind. Bottom line; if rope and chain are actually nearly the same, you sure arn't going to see a difference with chain grade, except in <10 knots winds. It's all about windage, foils, and rigging (bridles).

 

[GHA]

I don't think the light chains would make a difference for movement except in less than maybe 5 knots of wind as the slightest breeze will move our 10mm chain across the top of a sandy bottom. A gloopy mud one might be different.

Probably right.
Though in these chats no one ever mentions the psychological effects, like worrying your skinny chain's going to snap when you're getting bashed around an anchorage in an extended big blow. Very real.

And some sort of drag device might well help keeping the speed down a bit, a boat at 1.4Kts is almost twice as much energy to move somewhere else to stop the boat compared with 1Kt.

 

[thinwater]

... no one ever mentions the psychological effects, like worrying your skinny chain's going to snap when you're getting bashed around an anchorage in an extended big blow. Very real....

When Dyneema climbing slings first came out I remember my climbing partner saying "Great, now I have to trust my life to something even less substantial looking." He was both kidding and fully serious. And in time, we retired all of out nylon slings and replaced them with Dyneema, even though nylon is still available. Dyneema is lighter and more cut resistant. Maybe the same is true here; the math says it's better, but the heart is timid for no reason. A bigger chain will always feel stronger. But don't we trust 6mm Amsteel more than 25mm hemp?

 

[GHA]

When Dyneema climbing slings first came out I remember my climbing partner saying "Great, now I have to trust my life to something even less substantial looking." He was both kidding and fully serious. And in time, we retired all of out nylon slings and replaced them with Dyneema, even though nylon is still available. Dyneema is lighter and more cut resistant. Maybe the same is true here; the math says it's better, but the heart is timid for no reason. A bigger chain will always feel stronger. But don't we trust 6mm Amsteel more than 25mm hemp?

Good point. As a huge fan of dyneema. And load testing.
Though I can see if the dyneema is caught round a rock and slowly grinding itself into destruction. Also having worked with dyneema it raises other changes in design with the point loads being so much higher.

As ever, so many variables....

 

[RupertW]

When Dyneema climbing slings first came out I remember my climbing partner saying "Great, now I have to trust my life to something even less substantial looking." He was both kidding and fully serious. And in time, we retired all of out nylon slings and replaced them with Dyneema, even though nylon is still available. Dyneema is lighter and more cut resistant. Maybe the same is true here; the math says it's better, but the heart is timid for no reason. A bigger chain will always feel stronger. But don't we trust 6mm Amsteel more than 25mm hemp?

And as we know why anchoring is such an emotive subject - it literally keeps us awake at night - then getting people to change their minds (or changing our own) is so hard.

I know that I would be much better off with a modern anchor than an old Delta but that Delta has never dragged, once set, in 7 years. So almost every time I use it I can think back to a time when the waves, wind, or bottom was worse than now, but it still worked. And on the odd night it's the worst yet so that becomes the new emotional benchmark.

But I will change the anchor eventually and probably very quickly the first time it actually does drag.

 

[vyv_cox]

Is the wear mechanism mostly metal-metal, or are we really looking at metals forming chemical compounds with whatever's in the water, followed by these compounds wearing away?

I would guess that anchor chains might have a very different life to mooring chains, due to spending a lot of their time out of the water?

But there is a huge market for anchor chain which only spends a few hundred hours a year in the water.

It gets very complicated when a fluid enters the equation, even water. We were doing abrasion tests using certain types of coal and several scientific grades of sand. The rig was a standard test I think, basically a drum with two test specimens bolted to an arm driven by a spindle. Some of the coal tests were done with added distilled water, which gave totally different results from the dry ones.

 

[thinwater]

Good point. As a huge fan of dyneema. And load testing.
Though I can see if the dyneema is caught round a rock and slowly grinding itself into destruction. Also having worked with dyneema it raises other changes in design with the point loads being so much higher.

As ever, so many variables....

I was NOT suggesting Dyneema for anchoring. I didn't say anything about anchoring, I was talking about rock climbing. This was simply a musing about how we measure safety in the primitive part of our brain. Nothing more.

 

[GHA]

I was NOT suggesting Dyneema for anchoring. I didn't say anything about anchoring, I was talking about rock climbing. This was simply a musing about how we measure safety in the primitive part of our brain. Nothing more.

I didn't mention dyneema for anchoring either Though it might have sounded like that, just that it is different applications, you can see climbing strops, your chain is hidden introducing the unknown to worry about..

 

[Neeves]

Anyone have any guesses how much worse riding about the anchor is on a skinny chain compared to more normal size?

The trouble is one never compares like with like. Anyone using lightweight chain will use a 'better' snubber than someone using heavier chain. As we have moved down in chain size we have moved up in snubber efficiency. Becuase you use more snubber you have the chain deployed differently, bigger loop, much of which hangs directly in front of the bow (impacting hydro-resistance - if its important? (don't know).

The better snubber impacts the speed at which the dynamic snatches are managed by the snubber (a point you have made a number of times that no-one has picked up on).

I'm looking at wear or abrasion but there are a number of variables, like the snubber, that will impact how the chain wears that simply cannot be accommodated in a test - and you would not use the lightweight chain, 6mm, in the same way a 10mm.

Jonathan

 

[Neeves]

10x sounds a lot I'm not sure my lockers would cope.. Might be viable somewhere with not much tide.

Unless you are using 6mm chain already - downsizing will free up locker space - and then 10:1 becomes quite feasible.

Jonathan

 

[Neeves]

I think the long scope and veering is a very interesting question - there's quite a bit of momentum on our boat built up on a swing if we are at 5 or 8 to one compared to being in a crowded anchorage at 3:1 in a blow. In both cases we've never dragged so far but I have felt more nervous at the shock loads even with snubbers. This year I'm planning to use forks a bit more in those situations but if I'm the only person doing that then I could hit nearby boats on one anchor alone.

I don't think the light chains would make a difference for movement except in less than maybe 5 knots of wind as the slightest breeze will move our 10mm chain across the top of a sandy bottom. A gloopy mud one might be different.

If you still experience shock loads, at any windspeed or any scope than your snubbers are inadequate. If you get shock loads at 3:1 then I'd suggest you are over stressing them, even if you do not get shock loads, at better scopes. You need to use longer snubbers.

What are you using now.

I agree that the hydrodynamic effect of the rode, whatever it is made from, is not significant - when you are at the point when worry creeps into the back of your mind. It might impact at 5 knots - but that's not important.

Jonathan

 

[Neeves]

And as we know why anchoring is such an emotive subject - it literally keeps us awake at night - then getting people to change their minds (or changing our own) is so hard.

I know that I would be much better off with a modern anchor than an old Delta but that Delta has never dragged, once set, in 7 years. So almost every time I use it I can think back to a time when the waves, wind, or bottom was worse than now, but it still worked. And on the odd night it's the worst yet so that becomes the new emotional benchmark.

But I will change the anchor eventually and probably very quickly the first time it actually does drag.

Deltas 'set the same way as a Rocna or Korba or Excel. The toe and shackle point engage at roughly the same time and bury together. If you bury the shackle - you need to bury chain, progressively. If the chain is big it will reduce the ability to be buried and your anchor will set more shallow, I've tested it. A shallow anchor, set at, say 400kg, has the same hold as a deepset anchor (of whatever anchor size) - in a straight line - but the shallow anchor will have lower hold to veering (to a not straight line pull)

Buy your self a couple of metres of 6mm chain and place it between the end of the 10mm rode and the shackle. Try setting the anchor now - you might find it sets more easily and more deeply. Set the same anchor in the same place using the same revs and similar scope with your standard 10mm chain. I confidently suggest the anchor will set more deeply with the 6mm chain. Once you have satisfied your self of any changes, take the 6mm chain off.

Never use the 6mm chain for anchoring again - keep it to secure your dinghy, or a dog. Or wrap it round a rock 2 times for a shore line.

Jonathan

 

[RupertW]

If you still experience shock loads, at any windspeed or any scope than your snubbers are inadequate. If you get shock loads at 3:1 then I'd suggest you are over stressing them, even if you do not get shock loads, at better scopes. You need to use longer snubbers.

What are you using now.

I agree that the hydrodynamic effect of the rode, whatever it is made from, is not significant - when you are at the point when worry creeps into the back of your mind. It might impact at 5 knots - but that's not important.

Jonathan

I don't know why you would think there would be no shock load at 3:1 - it may not be much as not much momentum has built up but the usual chain tautening at the end of each swing still happens.

Re: snubbers I break protocol a bit by using sacrificial rubber snubbers on the first 3m of the snubber line - they are great for absorbing the really light shock loads as they stretch to up to twice their length and they are on a nylon rode which I then extend where possible in stronger winds so the nylon takes up more of the serious shock with the rubber snubbers now mostly at full stretch. Even with this punishment the rubble snubbers last a couple of seasons before snapping - which simply puts the load back onto the main nylon snubber.

 

[Neeves]

I was obviously not clear.

You will suffer from shock loads at 3:1 and if you suffer from shock loads at 3:1 with snubbers attached - your snubbers are inadequate. Under the same conditions of depth and wind those shock loads would reduce if you deployed more rode and then your snubbers might cope.

Sorry for any confusion

Jonathan

 

[RupertW]

I was obviously not clear.

You will suffer from shock loads at 3:1 and if you suffer from shock loads at 3:1 with snubbers attached - your snubbers are inadequate. Under the same conditions of depth and wind those shock loads would reduce if you deployed more rode and then your snubbers might cope.

Sorry for any confusion

Jonathan

I think we are just reading a slightly different meaning to shock loads. I am still considering them as (harmless) shock loads even if fully mitigated by weak and strong elastic combined snubber, whereas as your definition is that a fully absorbed shock load is not a shock load anymore. Your definition makes more sense but I don't now know what to call that mitigated shockload that (now) slowly tautens up the anchor chain and snubber and (now) slowly releases it.

 

[AntarcticPilot]

It gets very complicated when a fluid enters the equation, even water. We were doing abrasion tests using certain types of coal and several scientific grades of sand. The rig was a standard test I think, basically a drum with two test specimens bolted to an arm driven by a spindle. Some of the coal tests were done with added distilled water, which gave totally different results from the dry ones.

I think it is "especially water"! Water is very strange stuff, with all sorts of unusual properties compared with most liquids. I suppose we should be glad that it doesn't behave like liquid Helium!

 

[Neeves]

I think it is "especially water"! Water is very strange stuff, with all sorts of unusual properties compared with most liquids. I suppose we should be glad that it doesn't behave like liquid Helium!

Nor liquid Krypton